The present invention relates to a process and apparatus for depositing on a substrate a thin film of a compound having at least one cationic constituent and at least one anionic constituent.
More specifically, it relates to a process for the deposition of monocrystalline or polycrystalline, fault-free, compact layers or films having a homogeneous thickness and stoichiometric composition, which can be used for producing light-emitting diodes and screens, solar cells and components for optoelectronics, infrared detection and magnetic recording. It more particularly relates to the deposition of compounds of formula C.sub.m A.sub.n in which C represents a cation, A represents an anion which can be an oxyanion or a polyoxyanion, m is the valence of the anion and n the valence of the cation.
Among the presently known processes for the deposition of thin monocrystalline or polycrystalline films on a substrate, reference can be made to the processes using vapour phase deposition and those using deposition in solution.
The vapour deposition methods include vacuum depositing processes by thermal evaporation or electron bombardment, by cathodic sputtering, by pyrolysis, or by chemical deposition (CVD) under the effect of the temperature or a discharge under a high DC voltage or a high frequency, as well as deposition processes under ultra-high vacuum by epitaxy using molecular and/or atomic beams.
The methods of deposition in solution include deposition processes by chemical reaction and processes by electrolysis. Most of these processes make it possible to obtain compact, polycrystalline layers under good conditions, but they are not very suitable for producing layers of compounds of formula C.sub.m A.sub.n, in which A is an oxyanion or polyoxyanion. Moreover, it is difficult to check the stoichiometry of the deposited compounds and the thickness of the layer. Finally, the deposition of monocrystalline layers is virtually impossible to obtain with most of these processes. Thus, vacuum deposition processes using thermoevaporation or electron bombardment are difficult to perform and check, because the starting compounds used for this deposition partly or totally decompose hot into elements having different vapour pressures.
Thus, the stoichiometry of the deposited compounds is often difficult to reach and the thickness is difficult to check, particularly over large surfaces. In most cases, a polycrystalline deposit is obtained having pores and defects. Moreover, vacuum deposition processes cannot be used for depositing compounds such as phosphates, arsenates, chromates and carbonates.
Vapour phase chemical deposition processes (CVD) involve chemical reactions having complicated kinetics and thermodynamic equilibria, which often makes it necessary to synthesise and purefy particular organometallic compounds, which are often toxic, inflammable and difficult to handle. Moreover, the by-products of the reaction contaminate the deposit, particularly carbon, oxygen and halogens. As in the case of vacuum deposition, the thickness and uniformity of the thickness are difficult to check, because they are dependent on the kinetics of the reactions in surface and volume, the temperature uniformity at high temperatures and the hydrodynamics of the gasses. The layers obtained, which can be amorphous or polycrystalline, often have a porosity and epitaxial layers can only be deposited under special conditions aiding the surface heterogeneous autocatalysis, which generally requires the use of a hot substrate in a cold tube and causes problems with respect to the uniformity of temperature. In the same way, these processes are not suitable for the deposition of compounds of formula C.sub.m A.sub.n, in which A is an oxyanion or a polyoxyanion.
Deposition processes under ultra-high vacuum by molecular beams make it possible to obtain layers with a good crystalline quality, whose thickness and stoichiometry can be checked, which are easy to dope, but whose growth rate is low and it is also necessary to use complex, onerous equipment. Moreover, these processes are not suitable for the deposition of layers of compounds containing oxyanions or polyoxyanions.
Deposition processes by chemical reaction in solution, like that described in French Pat. No. 2 138 410 giving rise to double decompositions reactions in solution, are easy to perform between ambient temperature and 100.degree. C. and are also extremely economic. It is difficult to check the thickness of the layers, but the uniformity is good, the layers being obtained being stoichiometric and their doping is easy. However, this process can only be used with certain chalcogenides and metals, because it is difficult to find slow heterogeneous chemical reactions aiding the deposition coinciding with the homogeneous chemical reaction producing a precipitate.
Processes by electrolysis can only be carried out with certain conductive compounds, such as nickel sulphide or cobalt sulphide, or semiconductor compounds such as cadmium telluride. Deposition can be obtained by annodic oxydation, cathodic reduction or cathodic codeposition, but the thickness and stoichoimetry of the layers are difficult to check. Moreover, the layers often have cracks, incorporate impurities and it is not possible to obtain monocrystalline layers.
Thus, none of the known processes can lead to the obtaining of polycrystalline or monocrystalline layers of compounds of varied types, particularly compounds containing oxyanions or polyoxyanions.